Metadata: Set labels based on matching keywords in title and subject

[lastzero]

As a user with properly titled pictures that may also have subject metadata, I want the appropriate labels to be automatically assigned to them so I don't have to set them manually.

Labels should only be set automatically if the title was not generated automatically. Also, a cache should be used to find existing labels when indexing to improve performance.

Related Issues:

• Metadata: Set labels based on matching keywords in the caption #4603
• Labels: Improve the tagging experience #317
• Labels: Automatically translate label names #485
• Labels: Option to automatically create labels for keywords coming from photo metadata (Exif/IPTC/XMP) #1143